This invention relates to an asynchronous transfer mode (ATM) relay system and, more particularly, to a private branch exchange system or a switching system connected to an ATM network.
An ATM relay system of the type described is disclosed in a report which is contributed by Masaharu Morikawa et al. to NEC technical report, Vol. 48, No. 4 (1995), pages 46-51, and which has a title of "ATM Multiplex System MM-Node 9510". According to this report, the ATM relay system comprises an ATM network and a plurality of private branch exchange systems. Each private branch exchange system comprises a switching system and an ATM multiplexing unit connected to the ATM network. The switching system is connected to the ATM multiplexing unit via a standard private line interface. It will be assumed that relay connection is made from the ATM network to the ATM network. In this event, the relay connection is carried out via a synchronous transfer mode (STM) link in the switching system. Under the circumstances, the ATM multiplexing unit must carry out cell assembly and disassembly therein. In other words, the ATM multiplexing unit must carry out ATM/STM conversion.
However, the conventional ATM relay system has troubles as follows. The cell assembly and disassembly must be carried out for the ATM/STM conversion in the conventional ATM relay system comprising a plurality of switching systems whenever the relay connection is carried out via the plurality of switching systems. As a result, delay of the cell assembly occurs. The delay of the cell assembly has adverse effect in speech communication owing to generation of echoes or the like. In addition, there is a case where a high efficient encoding such speech compression, silent compression or the like is carried out in the conventional ATM relay system. When multistage ATM/STM conversions are carried out for the relay connection with such a high efficient encoding carried out, degradation of speech quality becomes violent in the speech communication. For example, a trouble of no communication arises in facsimile communication or the like.